Field of Invention
The present invention relates to a measuring device for natural gas content, and more particularly to an automatic measuring instrument and a measuring method for an unconventional natural gas content.
Description of Related Arts
At present, the measurement of the content of natural gas contained in a natural gas reservoir sample at home and abroad means to collect the content of natural gas contained in a given weight of sample placed in a sample parsing tank (namely a constant-volume airtight container) within a specified measurement time under the conditions of certain temperature (stimulated reservoir temperature) and pressure (generally atmospheric pressure). A tiny-pressure sensor gas content volume measurement method and a gas flowmeter measurement method are mainly adopted at home and abroad. The tiny-pressure sensor gas content volume measurement method lies in that tiny pressure generated when the natural gas enters an airtight measuring tube from the parsing tank is increased to make the liquid level in the measuring tube begin to drop from an upper limit value. The measurement of the natural gas content just means to read and record a volume value corresponding to a scale change value after the liquid level in the measuring tube drops to a lower limit value, and in the meantime, the vertical height (namely one end, connected to the atmosphere, of a U-shaped tube) of a liquid storage balance tube needs to be manually changed in order to keep the liquid level in the measuring tube to be consistent to the liquid level in the liquid storage balance tube (namely the tiny pressure difference is zero), so as to ensure the measurement accuracy of the gas content. A gas exhaust valve of the parsing tank is closed. A gas exhaust valve of the measuring tube is opened and the height of the liquid storage tube is adjusted to enable the liquid level in the measuring tube and the liquid level in the liquid storage tube to reach the upper limit values again. The gas exhaust valve of the measuring tube is closed and the gas exhaust valve of the parsing tank is opened again, and then next constant-volume measurement of gas content can be started. The tiny-pressure sensor gas volume measurement method needs real-time observation and record by manpower. It is large in manual operation and reading errors and high in labor intensity, and instruments are not easy to carry because of larger size, whereas the gas flowmeter measurement method means to directly connect a gas flowmeter to an outlet of the sample parsing tank to measure and automatically record the content of the natural gas separated out from the sample. The measurement accuracy of this method depends on the selected flowmeter and will be influenced since the content of the parsed natural gas contained in the reservoir sample is not flowing in a continuous constant-flow manner.
Unconventional natural gases, such as shale gas and coal bed gas, are important constituents of the world energy sources. With the continuous rise of world energy consumption, the unconventional natural gas energy sources, such as shale gas and coal bed gas, get more and more attention, and a development technology thereof is developed fast thereupon. In order to ensure the validity and reliability of results in calculation of development reserves of the unconventional natural gas and to reduce the manual operation error and alleviate the labor intensity of test work, there is a need for a novel instrument which can be used for directly and accurately measuring the content of natural gas (parsed or adsorbed natural gas) of an unconventional natural gas reservoir sample in a downhole temperature environment and has a function of continuous automatic constant-volume measurement for long-term and non-constant-flow gas output.